Particularly in broadband amplifiers and in power amplifiers, amplifier non-linearities cause distortion to an output signal. FIG. 1 illustrates the formation of intermodulation products (IM-products) causing distortion to an output signal. An undistorted input signal 12 consisting of two frequency components ω1 and ω2 is fed into an amplifier 14. At the output of the amplifier 14 appears a distorted signal 16, which includes desired signal components at frequencies ω1 and ω2 and also two strong undesired signal components close to the desired signal components at the frequencies 2*ω1–ω2 and 2*ω2–ω1. These undesired signals may appear in the desired signal band, and thereby corrupt the output signal.
LDMOS (Laterally Diffused Metal Oxide Semiconductor) devices are relatively new semiconductor devices, which have improved characteristics compared to conventional semiconductor devices. For example, they have improved IM performance, i.e. they produce less IM-products at the same input power than the conventional semiconductor devices. Typical input power/IM-products curves of a conventional semiconductor device and of an LDMOS device are illustrated in FIG. 2 by a dashed line and a solid line, respectively. In a conventional semiconductor device, the amount of IM-products increases as the input power increases. The advantage of an LDMOS device is that it has an optimal operating point called “sweet point” producing an optimal input power/IM-products ratio.
The characteristics of LDMOS devices make them especially well suited for power amplification. However, they are not widely used at the moment. A reason for this is a considerable drift in gate-source bias voltage, or Vgs. The Vgs is a DC voltage which is used to provide a certain drain-source quiescent current, or ldq. The location of the “sweet point” on the input power axis in FIG. 2 is dependent on this quiescent current. Therefore, the Vgs can be used to set the “sweet point”, i.e. optimal operating point, to the preferred input power level. However, if the Vgs is kept constant, the ldq decreases in the course of time because of aging of the LDMOS transistors. In other words, the Vgs corresponding to a certain ldq changes in the course of time. This kind of drift in bias voltage due to transistor aging also appears in “normal” semi-conductor devices, but it is a considerable problem with LDMOS transistors, in particular. Therefore, a method of keeping the ldq constant is needed for LDMOS devices in order to maintain their “sweet point” at the desired signal level. This object can be achieved by reducing the Vgs drift or by adjusting the Vgs for keeping the ldq constant.
One possible method of reducing the Vgs drift is to “burn in” LDMOS devices before they are used, because the drift phenomenon is most considerable at the beginning of the use of an LDMOS device. The disadvantage of this procedure is that it reduces the lifetime of the device. Additionally, “burn in” is difficult and expensive to implement.
Since the Vgs drift has been reduced from more than 25% to 5% for 900 MHz parts and to 12–15% for 2 GHz parts by device manufacturers, the problem of the drift has diminished. However, the problem has not disappeared, because a 10% drift may produce 5–10 dB IM-products, which is still relatively much in a linear amplifier.
One proposed method of adjusting the Vgs to keep the ldq constant is to measure the ldq and make Vgs adjustments on the basis of the measured ldq. The problem in this kind of an arrangement is that the amplifier has to be shut down during the measurement. Additionally, the transistor characteristics that vary from one transistor to another have an effect on the measurement results.